1. Field of the Invention
The subject invention is generally related to systems for delivering drilling fluid (or “drilling mud”) for oil and gas drilling applications. More particularly, the present invention is directed to a system for controlling the density and flow of drilling mud in offshore (deep and shallow water) and land-based oil and gas drilling applications.
2. Description of the Prior Art
It is well known to use drilling mud to provide hydraulic horse power for operating drill bits, to maintain hydrostatic pressure, to cool the wellbore during drilling operations, and to carry away particulate matter when drilling for oil and gas in subterranean wells. In basic operations, drilling mud is pumped down the drill pipe to provide the hydraulic horsepower necessary to operate the drill bit, and then it flows back up from the drill bit along the periphery of the drill pipe and inside the open borehole and casing. The returning mud carries the particles loosed by the drill bit (i.e., “drill cuttings”) to the surface. At the surface, the return mud is cleaned to remove the particles and then is recycled down into the hole.
The density of the drilling mud is monitored and controlled in order to maximize the efficiency of the drilling operation and to maintain hydrostatic pressure. In a typical application, a well is drilled using a drill bit mounted on the end of a drill string. The drilling mud is pumped down the drill pipe and through a series of jets in the drill bit to provide a hydraulic horsepower at the cutting bit face. The mud passes through the drill bit and flows upwardly along the drill string inside the annulus formed between the open hole or cased hole and the drill string, carrying the loosened particles to the surface.
Besides the density, the velocity or rate of the return mud flow must also be monitored and controlled. The rate at which the return mud flows upward through the annulus between the open/cased hole and the drill string is referred to as the “annular velocity.” The annular velocity of the return mud is commonly expressed in units of feet per minute (FPM) and is a function of the cross-sectional area of the annular space between the hole and the drill string. If this cross-sectional area is reduced, then the annular velocity of the return mud flowing through that area will naturally increase. Typically, this is problematic where the hole diameter is large—such as the surface casing hole. Typically the first borehole(s) drilled below the surface casing use tubing diameters ranging between 12″ and 18″. Since conventional drill strings are composed of drill pipes having an outer diameter ranging from 2⅞″ to 6⅝″, the annular space between the drill pipe and the wellbore is relatively large. This results in a slower annular velocity for return mud flowing through these zones.
The annular velocity of the return mud must be monitored for at least two important reasons. First, the annular velocity of the return mud must be maintained to be greater than the rate at which the cuttings and debris being carried by the mud slip downward due to the effects of gravity. This is referred to as “critical velocity.” If the annular velocity of the return mud falls below the critical rate, then there will be a risk that the cuttings and debris particles will slip and settle thus forming bridges that may obstruct the wellbore. Furthermore, the annular velocity of the return mud must be maintained at a laminar level to avoid turbulent flow which could be damaging to the formation itself, and also increase the equivalent circulating density unnecessarily.
One example of a mud control system is shown and described in U.S. Pat. No. 5,873,420, entitled “Air and Mud Control System for Underbalanced Drilling”, issued on Feb. 23, 1999 to Marvin Gearhart. The system shown and described in the Gearhart patent provides for a gas flow in the tubing for mixing the gas with the mud in a desired ratio so that the mud density is reduced to permit enhanced drilling rates by maintaining the well in an underbalanced condition.
It is known that there is a preexistent pressure on the formations of the earth, which, in general, increases as a function of depth due to the weight of the overburden on particular strata. This weight increases with depth so the prevailing or quiescent bottom hole pressure is increased in a generally linear curve with respect to depth. As the well depth is doubled in a normal-pressured formation, the pressure is likewise doubled. This is further complicated when drilling in deep water or ultra deep water because of the pressure on the sea floor by the water above it. Thus, high pressure conditions exist at the beginning of the hole and increase as the well is drilled. It is important to maintain a balance between the mud density and pressure and the hole pressure. Otherwise, the pressure in the formation will force material back into the wellbore and cause what is commonly known as a “kick.” In basic terms, a kick occurs when the gases or fluids in the wellbore flow out of the formation into the wellbore and migrate upward. When the standing column of drilling fluid is equal to or greater than the pressure at the depth of the borehole, the conditions leading to a kick are minimized. When the mud density is insufficient, the gases or fluids in the borehole can cause the mud to decrease in density and become so light that a kick occurs.
Kicks are a threat to drilling operations and a significant risk to both drilling personnel and the environment. Typically blowout preventers (or “BOP's”) are installed at the ocean floor or at the surface to contain the wellbore and to prevent a kick from becoming a “blowout” where the gases or fluids in the wellbore overcome the BOP and flow upward creating an out-of-balance well condition. However, the primary method for minimizing the risk of a blowout condition is the proper balancing of the drilling mud density to maintain the well in an overbalanced condition at all times. While BOP's can contain a kick and prevent a blowout from occurring thereby minimizing the damage to personnel and the environment, the well is usually lost once a kick occurs, even if contained. It is far more efficient and desirable to use proper mud weight control techniques in order to reduce the risk of a kick than it is to contain a kick once it occurs.
In order to maintain a safe margin, the column of drilling mud in the annular space around the drill stem is of sufficient weight and density to produce a high enough pressure to limit risk to near-zero in normal drilling conditions. This is referred to as “overbalanced” drilling. In an overbalanced state, the hydrostatic pressure induced by the weight of the drilling fluid is greater than the actual pore pressure of the formation. However, during overbalanced drilling, the drilling mud may penetrate the formation from the wellbore. Moreover, too much overbalanced drilling slows down the drilling process.
Alternatively, in some cases, underbalanced drilling has been attempted in order to increase the drilling rate and to reduce drilling mud penetration into the formation. In an underbalanced state, the hydrostatic pressure induced by the weight of the drilling fluid in the well is less than the actual formation pressure within the pore spaces of the formation. Accordingly, during underbalanced drilling, the fluids within the pore spaces of the reservoir formation actually flow into the wellbore. As such, underbalanced drilling presents significant benefits: (1) the rate of penetration or speed of well construction is increased, (2) the incidence of drill pipe sticking is decreased, and (3) the risk of losing expensive drilling into the formation is practically eliminated.
Furthermore, deep water and ultra deep water drilling has its own set of problems coupled with the need to provide a high density drilling mud in a wellbore that starts several thousand feet below sea level. The pressure at the beginning of the hole is equal to the hydrostatic pressure of the seawater above it, but the mud must travel from the sea surface to the sea floor before its density is useful. It is well recognized that it would be desirable to maintain mud density at or near seawater density (or 8.6 PPG) when above the borehole and at a heavier density from the seabed down into the well. In the past, pumps have been employed near the seabed for pumping out the returning mud and cuttings from the seabed above the BOP's and to the surface using a return line that is separate from the riser. This system is expensive to install, as it requires separate lines, expensive to maintain, and very expensive to run. Another experimental method employs the injection of low density particles—such—as glass beads into the returning fluid in the riser above the sea floor to reduce the density of the returning mud as it is brought to the surface. Typically, the BOP stack is on the sea floor and the glass beads are injected above the BOP stack.
While it has been proven desirable to control drilling mud density and flow in a wellbore, during the drilling of oil and gas wells there are no prior art systems that effectively accomplish this objective. The present invention provides such a system.